The present invention relates to a method of controlling the actuation of emergency braking in a motor vehicle having a brake system in which the braking force determined by the position of the vehicle brake pedal is augmented when the actuation speed of the brake pedal exceeds a threshold value.
Braking processes of motor vehicles are initiated by an actuation of the brake pedal, which causes braking of the motor vehicle in proportion to the pedal position of the brake pedal. In emergency situations, very rapid and sharp braking of the motor vehicle is required to shorten the braking distance; however, this objective frequently cannot be achieved by the driver of the motor vehicle. It has therefore been proposed to detect an emergency situation from the manner in which the driver treads on the brake pedal (the pedal actuation speed) and to take suitable supportive measures in this case. That is, if the pedal actuation speed exceeds a certain specified threshold triggering value, an emergency situation is detected, and additional brake pressure is fed to the brake cylinders via a solenoid valve. In this manner, additional braking force is applied ("full braking") and insufficiently strong actuation of the brake pedal is thus compensated.
The disadvantage of such systems as described above is that variations in the properties of the brake system between different vehicles (e.g. due to tolerances across a model series) on the one hand, and changes in the response characteristics of the brake system in a particular motor vehicle (e.g., due to aging, brake lining wear etc.) on the other hand, are not taken into account in the determination of the trigger threshold value for the detection of an emergency situation. In practical operation, this results in a different trigger threshold value input in the case of different motor vehicles, or a shift in the trigger threshold value on the same motor vehicle, and hence in a different and often unacceptable assessment of emergency situations, frequently leading to excessively early switching in of the solenoid valve.
The object of the present invention is to provide an improved method for controlling the actuation of emergency braking which avoids these disadvantages.
This object is achieved according to the invention by using a threshold value for triggering automatic braking which takes into account not only of the travelling speed of the motor vehicle and of the position of the brake pedal, but also a "correction value" which reflects the effect of the individual properties of the brake system of the respective motor vehicle. To arrive at the correction value, during "normal" braking processes (i.e., in the range of low-level braking, not ABS braking), the braking force input is determined (e.g., by evaluation of the brake pedal travel or of the diaphragm travel of the brake booster), and the instantaneously acting braking force is determined as well (e.g., by determining the braking retardation by means of wheel speed sensors). A braking force transmission ratio is then calculated as the quotient of the specified braking force and the instantaneously acting braking force. This braking force transmission ratio is compared with a fixed predetermined standard value to determine a "relative value", being the quotient of the current braking force transmission ratio and the standard value. The correction value is then determined as a function of the relative value in such a manner, described hereinafter, that the input of the trigger threshold value always results in the same triggering behavior.
The values determined for the correction value are stored, and used to determine the trigger threshold value during subsequent braking processes. In the most general case, the time variation of the relative value during the braking process is taken into account, and the required correction value is determined by reference to a family of characteristics, which vary with time during the braking process, dependent on the relative value. In addition, it is possible, during a braking process, to determine an average relative value of the braking force transmission ratio, and to assign to this ratio correction value taken from a characteristic which reflects the relationship between the correction value and the relative value averaged over the course of the braking process. The time dependent or time independent (average) relative values of different braking processes can furthermore be stored and evaluated (e.g., for long term consideration or in order to detect changes).
The relationship between the respective relative values and the associated correction values is generally nonlinear; often, however, as a practical matter it is satisfactory to assume a linear relationship between the relative value determined and the required correction value. In the simplest case, in fact, the correction value corresponds exactly to the relative value.
The trigger threshold value is advantageously constant for one motor vehicle, and hence also for different boundary conditions and ambient conditions and for different motor vehicles of a model series, even in the case of different brake systems. This means that the assessment of an emergency situation is always identical, which increases both driving safety, and the degree of acceptance of the method by the user of the motor vehicle.